In the field of graphic arts, printing of a printing plate is performed with a set of color separation films formed from a color original by a lith film. In general, color proofs are formed from color separation films before actual printing work for checking an error in the color separation step and the necessity for color correction. Color proofs are desired to realize high definition which makes it possible to surely reproduce a half tone image and have performances such as high stability of processing. Further, for obtaining color proofs closely approximating to an actual printed matter, it is preferred to use materials which are used in actual printing as the materials for making color proofs, e.g., the actual printing paper as the base material and pigments as the coloring materials. As the method for forming a color proof, a dry method not using a developing solution is strongly desired.
As the dry method for forming color proofs, a recording system of directly forming color proofs from digital signals has been developed with the spread of electronized system in preprocessing of printing (pre-press field) in recent years. Such electronized system aims at forming in particular high quality color proofs, generally reproducing a dot image of 150 lines/inch or higher. For recording a proof of high image quality from digital signals, laser beams capable of modulation by digital signals and capable of finely diaphragming recording lights are used as recording heads. Therefore, the development of an image-forming material having high recording sensitivity to laser beams and exhibiting high definition property capable of reproducing highly minute dots is required.
As the image-forming material for use in a transfer image-forming method using laser beams, a heat fusion transfer sheet comprising a support having thereon in the order of a photothermal converting layer which absorbs laser beams and generates heat, and an image-forming layer which contains a pigment dispersed in components such as a heat fusion type wax and a binder is known (JP-A-5-58045 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)). In the image-forming method using such an image-forming material, an image-forming layer corresponding to the area of a photothermal converting layer irradiated with laser beams is fused by heat generated in that area and transferred onto an image-receiving sheet arranged on the transfer sheet by lamination, thus a transferred image is formed on the image-receiving sheet.
Further, a thermal transfer sheet comprising a support having provided thereon in the order of a photothermal converting layer containing a light-to-heat converting material, an extremely thin heat-releasing layer (from 0.03 to 0.3 μm), and an image-forming layer containing a coloring material is disclosed in JP-A-6-219052. In the thermal transfer sheet, the bonding strength between the image-forming layer and the photothermal converting layer bonded through the intervening heat-releasing layer is reduced by laser beam irradiation, as a result, a highly minute image is formed on an image-receiving sheet arranged on the thermal transfer sheet by lamination. The image-forming method by the thermal transfer sheet utilizes so-called ablation, specifically the heat-releasing layer partially decomposes at the area irradiated with laser beams and vaporizes, thereby the bonding strength of the image-forming layer and the photothermal converting layer at the irradiated area is reduced and the image-forming layer at that area is transferred to the image-receiving sheet laminated thereon.
These image-forming methods have advantages such that an actual printing paper provided with an image-receiving layer (an adhesion layer) can be used as the material of an image-receiving sheet, and a multicolor image can be easily obtained by transferring images different in colors in sequence on the image-receiving sheet. In particular, the image-forming method utilizing ablation has the advantage such that highly minute image can be easily obtained, and so these methods are useful for forming a color proof (DDCP: direct digital color proof) or a highly minute mask image.
DTP is prevailing more and more and the intermediate process of using films is omitted when CTP (computer to plate) is used, and the need for proof is shifting from analog proof to DDCP. In recent years the demand for large sized high grade DDCP which is highly stable and excellent in coincidence in printing has increased.
High definition printing can be effected according to a heat transfer method by laser irradiation, and as the laser heat transfer methods, (1) a laser sublimation method, (2) a laser ablation method, and (3) a laser fusion method are conventionally used, but any of these methods has a drawback such that the shape of a recorded dots are not sharp. In (1) a laser sublimation method, since dyes are used as the coloring material, the approximation of proofs to printed matters is not sufficient, further, since this is a method of sublimating coloring materials, the outline of a dot is fuzzy, and so definition is not sufficiently high. On the other hand, since pigments are used as the coloring materials in (2) a laser ablation method, the approximation to printed matters is good, but since this is a method of sputtering coloring materials, the outline of a dot is also fuzzy as in the sublimation method, and so definition is not sufficiently high. Further, in (3) a laser fusion method, since a molten substance flows, the outline of a dot is not also clear.